Issues

outgoing email

followed the instructions, yet those postfix setup, from fields is <username>@<hostname>, which MailJet will send a verification email to, and apparently it won’t be verified.
https://app.mailjet.com/account/sender

additional steps to modified the email address, so it passes the verification.
https://www.cyberciti.biz/tips/howto-postfix-masquerade-change-email-mail-address.html